The present invention relates to an optical recording disc on which an information is optically recorded and/or reproduced.
The present invention relates particularly to an optical recording disc which can reduce a size of an optical recording/reproducing apparatus using the optical recording disc and can provide a more accurate information recording and reproduction with a strain generated in a disc main body extremely reduced.
In an information recording disc drive apparatus which carries out a record/reproduction of information with an information recording disc such as a random access type optical disc or a magneto-optical disc, a turn table is usually rotated at an extremely high speed. Therefore, a slip between the turn table and information recording disc will occur in such a case that the information recording disc is merely mounted on the turn table.
In order to prevent the occurrence of slip, a transferable chuck member is installed which grasps the information recording disc with the turn table when it is moved in a direction of the rotation axle of the turn table so that the information recording disc is firmly grasped.
However, if the transferable chuck member is installed, a space for a movement stroke of the transferable chuck member must be provided in a thickness direction of the information recording disc drive apparatus. Therefore, the information recording disc drive apparatus cannot sufficiently be miniaturized.
In order to prevent the occurrence of slip and reduce a size of the information recording disc drive apparatus, such an information recording disc drive apparatus has been proposed that a magnetic circular plate is fixed on the information recording disc and the magnetic circular plate is magnetically attracted to the turn table to hold the information recording disc.
FIGS. 1 through 3 show a turn table for the information recording disc having the construction described above.
An information recording disc generally denoted by m comprises a disc main body a including a substrate formed of a thermoplastic synthetic resin on which a predetermined record layer and its protective layer are formed. The information recording disc is, for example, an optical disc.
Located below the information recording disc main body a is a magnetic circular plate b having a centering hole c penetrated therethrough at a center part of the magnetic circular plate b. The magnetic circular plate b is made of a magnetic material and is formed in a circular shape. A hub d is integrally formed with the magnetic circular plate b along an outer peripheral edge thereof. The hub d is circular and is made of a thermoplastic synthetic resin, preferably of the same material as the information recording disc main body a or the same series material. An expansion molding rib e is projected on an upper surface of the hub d.
The hub d is fixed to a lower surface of the information recording disc main body a by means of a supersonic wave expansion molding so that the magnetic circular plate b is fixed to a lower surface of the information recording disc main body a via the hub d. It is noted that the centering hole c formed so as to penetrate through the magnetic circular plate b is positioned so as to match with the center of the information recording disc main body a.
A turn table generally denoted by f includes: a drive shaft g; a thick main body part h fixed around an upper end of the drive shaft g; and a magnet fixed to the thick main body part h.
The turn table f is rotatable by means of a suitble drive mechanism and an upper end thereof is formed in a rounded circular truncated cone shape. In addition, a circular recess j is formed on an upper surface of the turn table main body part h. A circular yoke plate k and circular magnet i are housed in this order and fixed to the circular recess j. Furthermore, a disc receiving surface .lambda. is formed at an outside of the recess j and has an inner diameter larger than an outer diameter of the hub d.
When the information recording disc m is mounted on the turn table f, the magnetic circular plate b attached to the information recording disc m is attracted to the magnet i of the turn table f, so that the upper end of the drive shaft g becomes engaged with the centering hole c provided through the magnetic circular plate b and a lower surfac at the outside of the hub d of the information recording disc m is mounted on the disc receiving surface .lambda..
Hence, the centering of the information recording disc m is carried out when the upper end of the drive shaft g is engaged with the centering hole c of the magnetic circular plate. In addition, a record/reproduction positioning of the information recording disc is carried out by receiving the lower surface at the outside of the hub d of the information recording disc m.
Since in the turn table f described above, such a member as the transferable chuck member described above which grasps the information recording disc when it is moved in the thickness direction of the information recording disc in order to securely hold the information recording disc main body a is not required, the size of the information recording disc drive apparatus, especially, the size thereof in its thickness direction can accordingly be reduced.
However, such a problem arises that a strain will occur in the substrate of the information recording disc m described above so that an inaccurate record/read of information will result.
That is to say, since the hub d which holds the magnetic circular plate b is fixed to the disc main body a by means of the thermal expansion molding, a stress is imposed on the substrate of the disc main body a and causes the generation of stress on the substrate. In addition, in an information record/playback apparatus using such an information recording disc, a temperature within the apparatus reaches as high as 50.degree. C. or more during operation so that a temperature difference from a normal temperature becomes 20.degree. C. or more. Consequently, a stress in a tensile direction (tensile stress) is imposed on the center part of the substrate of the disc main body due to the difference in thermal expansion rates between the substrate of the information recording disc and magnetic circular plate. On the other hand, a stress in a compressive direction (compressive stress) is imposed on a part outside of a part of the disc main body at which the hub d is fixed to the substrate of the disc main body by means of an expansion molding.
Furthermore, if the information recording disc m is a magneto-optical disc, the above-described stresses cause a birefrigence and therefore reduces a property of the optical recording disc. Particularly, since a minute rotation of a polarized surface of light is read as a signal in a case of a magneto-optical disc recording system, the birefrigence makes the signal quality remarkably deteriorated. In addition, since a light elastic modulus of a polycarbonate resin is large in a case when the polycarbonate resin is used as the material of the disc substrate, the increase rate in the birefrigence is large due to the temperature difference described above as compared with an acrylic resin or glass which is used as the material of the substrate of the disc main body. Specifically, the increase rate of the birefrigence value is 40 nm or more. Consequently, it is impractical.
FIG. 13(C) is a graph representing the birefrigence in the above-described information recording disc. A vertical axis in FIG. 13(C) denotes the birefrigence in a unit of nm when a laser beam is reciprocated and a horizontal axis therein denotes a distance in a unit of mm from the center of the information recording disc m in a radial direction. A curve A represents the birefrigence of the disc main body a before the hub d is fixed to the substrate by means of the thermal expansion molding, a curve B represents the birefrigence thereof after the hub d is fixed to the disc main body by means of the thermal expansion molding at an ambient temperature of 20.degree. C., and a curve C represents the birefrigence of the disc main body a when the hub is fixed to the disc main body by means of the thermal expansion molding and thereafter the ambient temperature is increased to 50.degree. C., respectively. It is noted that the information recording disc used to obtain measured values shown in FIG. 13(C) includes: the substrate made of the polycarbonate resin having the thickness of 1.2 mm and outer diameter of 130 mm; the hub made of the polycarbonate resin having the outer diameter of 32 mm and the thickness of 2 mm; and the magnetic circular plate made of a magnetic stainless SUS 430 having the thickness of 0.5 mm and which is press punched. In addition, in FIG. 13(C) a direction of minus sign (-) denotes a direction of the birefrigence generated due to the radial directional stress.
As appreciated from FIG. 13(C), the birefrigence is changed about 100 nm from the curve A to the curve B at a position 30 mm away in the radial direction from the center of the most inner periphery of a record area of the information recording disc. These measurement results show that the information recording disc drive apparatus having the turn table and the information recording disc described above cannot almost be reduced in a practical use.